Conventional methods and apparatus determine aeration of a fluid at one location in an engine system. The aeration value determined is indicative of the amount of air present in the fluid at that location. U.S. Published application Ser. No. 12/899,670 relates to a method and apparatus for measuring oil aeration of an engine. A density of pure oil and a density of air are calculated based on a measured oil pressure and a measured oil temperature of an oil line connecting a hydraulic pump of an engine and an oil gallery of a cylinder block. The oil aeration is then calculated based on a measured oil density, the measured oil pressure, the calculated density of the pure oil, and the calculated density of the air.